The initial commercial introduction of magnetically based electronic article surveillance (EAS) systems over twenty years ago included the use of a marker formed of a strip of permalloy about seven inches long, adopted to be concealed within the spine of a book, adhered between pages, etc. The strip was basically detectable only in one direction, hence various techniques were developed to overcome that limitation. Some were directed to the markers themselves, such as the use of more than one marker, positioned at right angles, L or X shaped markers, etc.
Still other techniques were directed to providing interrogation fields extending in various directions such that the markers could be detected regardless of their orientation. Thus, for example, U.S. Pat. Nos. 3,665,449 and 3,697,996 (Elder and Wright) disclose the use of three coils positioned to generate fields in three orthogonal directions, together with electronic circuitry to sequentially energize each of the coils, thereby generating spatially separated fields, each of which extended primarily in one direction so as to enhance the detection of markers oriented so as to be detectable in that direction.
U.S. Pat. No. 4,135,183 (Heltemes) is directed to a different way of providing multidirectional detection. In that patent, it is proposed that complex, hence expensive, systems requiring sequential energization be avoided by providing a pair of coils, each of which is substantially planar, positioned on opposite sides of a corridor defining an interrogation zone therebetween. Both coils have substantially the same overall shape, and are wound in either a Figure-8 or hour-glass configuration. Such coils are said to produce fields that vary significantly in different regions and, thereby, enhance the detectability of markers regardless of orientation in the zone.
Other techniques for providing fields extending in different directions throughout the interrogation zone to enhance the detection of unidirectionally responsive markers regardless of orientation in the zone are discussed in U.S. Pat. Nos. 4,309,697 (Weaver); 4,326,198 (Novikoff); and 4,623,877 (Buckens).
The '697 patent proposes the use of a pair of lattice assemblies positioned parallel with each other, on opposite sides of an interrogation zone extending therebetween. A rhomboid-shaped transmitting, i.e., field-producing, coil is to be positioned within each of the lattices, with the diagonal side of each coil being oppositely directed; e.g., the coil on one side of the zone has its diagonal side directed upward, while the diagonal side of the other coil is directed downward with respect to a desired passageway through the zone. In that patent, it is further proposed that a lazy-8 receiver coil also be positioned parallel with and alongside each transmitting coil.
In contrast, one embodiment depicted in the '198 patent proposes the use of a pair of transmitting coils in one lattice assembly on one side of the zone, and a pair of receiving coils in another lattice assembly positioned parallel with the first lattice, but located on the other side of the zone. In that embodiment, the transmitting coils are in substantially the same plane, are offset both horizontally and vertically, and are connected so that current flows in the same direction in both coils. That embodiment also requires the use of similarly offset DC energized bias coils. While the vertical and horizontal offset facilitates the production of differently directed field components throughout the zone, it requires the use of dual, different lattice assemblies.
The '877 patent depicts another variant. In that patent, a pair of field coils and a pair of receiving coils are all enclosed in a single lattice. The field-producing coils are basically rectangular, with smaller rectangular coils being centered within a larger, more square one. The coils are connected so that current flows in the same direction in both. The lattices are used in pairs on opposite sides of the interrogation zone, and are connected so that current in the coils on one side flows in the opposite direction from that in the coils on the other side, when all are viewed from the same side of the zone.
A lattice assembly bearing some similarity to that adapted to enclose the coil assembly of the present invention is also set forth in U.S. Pat. No. 4,994,939, however, the coil assembly contained within the lattice assembly is not configured to provide extended, multidirectional detection throughout an interrogation zone.